At the Large Hadron Collider (LHC), heavy ions are accelerated to extremely high energies, which creates strong electromagnetic fields. As a result, photons from the oncoming lead-ion beams can interact with each other or with the nuclei; these interactions are known as ultraperipheral collisions. Photon–nucleus scatterings at the highest energy that can be achieved with existing particle accelerators are useful probes that allow physicists to investigate the structure of nuclei. While the common picture of nucleons is that they contain three quarks (up–up–down for protons and up–down–down for neutrons), in reality, a complex sea consisting of quark–antiquark pairs and gluons makes up a large fraction of the proton and neutron energies. Ultraperipheral collisions are an extraordinary tool to test the nature of nuclear matter.

The CMS experiment has recently released the first results using data from the first heavy-ion run of LHC Run 3. The results measure the production of D0 mesons (containing a charm quark and an up antiquark) and their antiparticles, D0 bar mesons (made of an up quark and a charm antiquark), in ultraperipheral collisions for the first time. D0 mesons are formed by charm quarks that are kicked out of the nuclei by the photons and carry information about parton distribution functions, which describe how quarks and gluons behave inside nuclei.

To measure D0 production, the CMS detector first selects events in which photon and lead nuclei collisions have caused the latter to break up. When this happens, neutrons flow from the collision in parallel to the beam, whereas protons and intact nuclei will follow a curved path as their charge interacts with the LHC’s magnetic fields. Two calorimeters, at zero degrees to the beam and located 140 m away on either side of the interaction point, are able to detect such neutrons. If they are seen in one calorimeter and not the other, in a time window consistent with the collision, this event is selected for further investigation.

Then, the products of the D0 decay – oppositely charged kaon and pion pairs – are reconstructed in the CMS detector. Physicists consider all combinations of pion and kaon trajectories, with each track taking an assumed mass of the kaon and pion. They then filter these combinations using the data to identify tracks that match what they expect from a D0 meson. From this, they are able to measure the so-called production cross section, which is the rate at which D0 mesons are produced.

For CMS, the study of nuclear structure using D0 meson production is just one of many applications of ultraperipheral collisions. With time, as methods are refined and systematic uncertainties are reduced, this technique will be able to constrain the parton distribution functions, allowing physicists to understand the structure of nuclear matter more deeply.

The next heavy-ion run of LHC Run 3 will start at the beginning of November.

Read more:

• CMS briefing
• Paper

The Estonian flag was raised today at a ceremony held at CERN to mark the country’s accession as CERN’s 24th Member State. The ceremony was attended by the President of the Republic of Estonia, Alar Karis, one of the two Vice-Presidents of the CERN Council, Eric Laenen, and the CERN Director-General, Fabiola Gianotti, together with representatives of CERN’s Member and Associate Member States and Observers and the CERN community.

“I am delighted that the aspirations of our academic community, companies and government have come together and made CERN membership a reality. This is a major milestone for Estonia and we are keen to contribute to the exciting research and innovation at CERN,” said Alar Karis.

“On behalf of the CERN Council, I am very pleased to welcome Estonia as a Member State, building on decades of commitment to basic science and peaceful scientific collaboration. We are now looking forward to reinforcing Estonia’s participation in the CERN Council,” said Eric Laenen.

“The raising of the Estonian flag is a highly symbolic culmination of Estonia’s journey to becoming a CERN Member State, which recognises the long-standing fruitful scientific cooperation between Estonia and CERN and Estonia’s significant contribution to fundamental research. We look forward to enhancing our collaboration in particle physics research, technology and innovation and in the training and education of the next generations with Estonia as a Member State,” said Fabiola Gianotti.

CERN has revised its schedule for its accelerator complex. The Large Hadron Collider (LHC) is currently in its third period of physics data taking (Run 3), and this will now be extended until July 2026. The third Long Shutdown (LS3) will then begin, seven and a half months later than originally planned. CERN’s other accelerators will continue to operate throughout July and August 2026 and will start their extensive programme of work in September 2026. 

The aim of LS3 is to prepare for High-Luminosity LHC (HL-LHC), when the LHC and its experiments will operate at higher luminosity, i.e. with an increased number of collisions in the LHC experiments. HL-LHC requires the installation of new key equipment, including more powerful focusing magnets, new superconducting cavities known as “crab cavities”, a reinforced protection system and innovative superconducting electrical transmission lines linking equipment in specially built technical galleries to the new magnets in the LHC tunnel. LS3 is expected to last around four months longer than initially planned, to allow civil engineering work to be carried out to connect the new HL-LHC technical galleries to the LHC tunnel.

During LS3, the ATLAS and CMS experiments will replace a large part of their electronic systems and many of their detectors, in particular their trackers. The delayed start of LS3 will allow the ATLAS and CMS collaborations more time to develop and build these highly sophisticated detectors and systems.

In addition to high-luminosity enhancements, LS3 will involve a huge programme of work across the facilities, including the first phase of consolidation of the North Area fed by the SPS accelerator, which hosts numerous fixed target experiments, and improvements to the ISOLDE nuclear physics facility and the facility for AWAKE, the plasma wakefield acceleration research programme.

The start-up of the HL-LHC (Run 4) is now planned for June 2030.